Added ignoreStackAndRegisters parameter to GC_step

doc/gc.txt

@@ -56,7 +56,7 @@ file as well). With this switch the GC supports the following operations:
 
 .. code-block:: nim
   proc GC_setMaxPause*(MaxPauseInUs: int)
-  proc GC_step*(us: int, strongAdvice = false)
+  proc GC_step*(us: int, strongAdvice, ignoreStackAndRegisters = false)
 
 The unit of the parameters ``MaxPauseInUs`` and ``us`` is microseconds.
 
@@ -75,7 +75,11 @@ These two procs are the two modus operandi of the realtime GC:
     This allows the GC to perform some work for up to ``us`` time. This is
     useful to call in a main loop to ensure the GC can do its work. To
     bind all GC activity to a ``GC_step`` call, deactivate the GC with
-    ``GC_disable`` at program startup.
+    ``GC_disable`` at program startup. Notice that you may ask GC to not
+    scan stack and registers for references via ``ignoreStackAndRegisters``
+    parameter. This may reduce the step time depending on the stack depth,
+    but use it only when you are sure that neither the stack nor the registers
+    contain unique references to objects that must be preserved.
 
 These procs provide a "best effort" realtime guarantee; in particular the
 cycle collector is not aware of deadlines yet. Deactivate it to get more

lib/system/gc.nim

@@ -905,19 +905,19 @@ proc unmarkStackAndRegisters(gch: var GcHeap) =
     #sysAssert c.typ != nil, "unmarkStackAndRegisters 2"
   gch.decStack.len = 0
 
-proc collectCTBody(gch: var GcHeap) =
+proc collectCTBody(gch: var GcHeap, ignoreStackAndRegisters = false) =
   when withRealTime:
     let t0 = getticks()
   sysAssert(allocInv(gch.region), "collectCT: begin")
-
-  when not defined(nimCoroutines):
-    gch.stat.maxStackSize = max(gch.stat.maxStackSize, stackSize())
   sysAssert(gch.decStack.len == 0, "collectCT")
   prepareForInteriorPointerChecking(gch.region)
-  markStackAndRegisters(gch)
-  markThreadStacks(gch)
-  gch.stat.maxStackCells = max(gch.stat.maxStackCells, gch.decStack.len)
-  inc(gch.stat.stackScans)
+  if not ignoreStackAndRegisters:
+    when not defined(nimCoroutines):
+      gch.stat.maxStackSize = max(gch.stat.maxStackSize, stackSize())
+    markStackAndRegisters(gch)
+    markThreadStacks(gch)
+    gch.stat.maxStackCells = max(gch.stat.maxStackCells, gch.decStack.len)
+    inc(gch.stat.stackScans)
   if collectZCT(gch):
     when cycleGC:
       if getOccupiedMem(gch.region) >= gch.cycleThreshold or alwaysCycleGC:
@@ -927,7 +927,8 @@ proc collectCTBody(gch: var GcHeap) =
         gch.cycleThreshold = max(InitialCycleThreshold, getOccupiedMem() *
                                  CycleIncrease)
         gch.stat.maxThreshold = max(gch.stat.maxThreshold, gch.cycleThreshold)
-  unmarkStackAndRegisters(gch)
+  if not ignoreStackAndRegisters:
+    unmarkStackAndRegisters(gch)
   sysAssert(allocInv(gch.region), "collectCT: end")
 
   when withRealTime:
@@ -971,16 +972,17 @@ when withRealTime:
   proc GC_setMaxPause*(MaxPauseInUs: int) =
     gch.maxPause = MaxPauseInUs.toNano
 
-  proc GC_step(gch: var GcHeap, us: int, strongAdvice: bool) =
+  proc GC_step(gch: var GcHeap, us: int, strongAdvice: bool, ignoreStackAndRegisters: bool) =
     acquire(gch)
     gch.maxPause = us.toNano
     if (gch.zct.len >= ZctThreshold or (cycleGC and
         getOccupiedMem(gch.region)>=gch.cycleThreshold) or alwaysGC) or
         strongAdvice:
-      collectCTBody(gch)
+      collectCTBody(gch, ignoreStackAndRegisters)
     release(gch)
 
-  proc GC_step*(us: int, strongAdvice = false) = GC_step(gch, us, strongAdvice)
+  proc GC_step*(us: int, strongAdvice, ignoreStackAndRegisters = false) =
+    GC_step(gch, us, strongAdvice, ignoreStackAndRegisters)
 
 when not defined(useNimRtl):
   proc GC_disable() =

lib/system/gc2.nim

@@ -894,18 +894,18 @@ proc unmarkStackAndRegisters(gch: var GcHeap) =
     decRef(d[i])
   gch.decStack.len = 0
 
-proc collectCTBody(gch: var GcHeap) =
+proc collectCTBody(gch: var GcHeap, ignoreStackAndRegisters = false) =
   when withRealTime:
     let t0 = getticks()
   sysAssert(allocInv(gch.region), "collectCT: begin")
-
-  when not defined(nimCoroutines):
-    gch.stat.maxStackSize = max(gch.stat.maxStackSize, stackSize())
   sysAssert(gch.decStack.len == 0, "collectCT")
   prepareForInteriorPointerChecking(gch.region)
-  markStackAndRegisters(gch)
-  gch.stat.maxStackCells = max(gch.stat.maxStackCells, gch.decStack.len)
-  inc(gch.stat.stackScans)
+  if not ignoreStackAndRegisters:
+    when not defined(nimCoroutines):
+      gch.stat.maxStackSize = max(gch.stat.maxStackSize, stackSize())
+    markStackAndRegisters(gch)
+    gch.stat.maxStackCells = max(gch.stat.maxStackCells, gch.decStack.len)
+    inc(gch.stat.stackScans)
   if collectZCT(gch):
     when cycleGC:
       if getOccupiedMem(gch.region) >= gch.cycleThreshold or alwaysCycleGC:
@@ -914,7 +914,8 @@ proc collectCTBody(gch: var GcHeap) =
           gch.cycleThreshold = max(InitialCycleThreshold, getOccupiedMem() *
                                    CycleIncrease)
           gch.stat.maxThreshold = max(gch.stat.maxThreshold, gch.cycleThreshold)
-  unmarkStackAndRegisters(gch)
+  if not ignoreStackAndRegisters:
+    unmarkStackAndRegisters(gch)
   sysAssert(allocInv(gch.region), "collectCT: end")
 
   when withRealTime:
@@ -949,14 +950,15 @@ when withRealTime:
   proc GC_setMaxPause*(MaxPauseInUs: int) =
     gch.maxPause = MaxPauseInUs.toNano
 
-  proc GC_step(gch: var GcHeap, us: int, strongAdvice: bool) =
+  proc GC_step(gch: var GcHeap, us: int, strongAdvice, ignoreStackAndRegisters: bool) =
     gch.maxPause = us.toNano
     if (gch.zct.len >= ZctThreshold or (cycleGC and
         getOccupiedMem(gch.region)>=gch.cycleThreshold) or alwaysGC) or
         strongAdvice:
-      collectCTBody(gch)
+      collectCTBody(gch, ignoreStackAndRegisters)
 
-  proc GC_step*(us: int, strongAdvice = false) = GC_step(gch, us, strongAdvice)
+  proc GC_step*(us: int, strongAdvice, ignoreStackAndRegisters = false) =
+    GC_step(gch, us, strongAdvice, ignoreStackAndRegisters)
 
 when not defined(useNimRtl):
   proc GC_disable() =